a) Field of the Invention
The present invention is directed to an optical imaging system for inspection microscopes with which lithography masks can be checked for defects particularly through emulation of high-aperture scanner systems.
b) Description of the Related Art
As object structures continue to decrease in size, increasingly higher image-side numerical apertures of scanner systems are required. However, the incident angles which are therefore also increasingly greater result in vector effects, as they are called, in which tangentially polarized and radially polarized beam components have different intensity distributions. It has been shown that the beam components which oscillate parallel to the incident plane defined by the incidence direction and surface normal of the substrate (s-polarized) interfere better and accordingly generate a better contrast than the beam components oscillating perpendicular thereto (p-polarized). These vector effects lead to a decreasing contrast for p-polarized components of radiation and, therefore, to a decreasing total contrast in scanner systems with high image-side numerical apertures.
The following description is based on this definition: Light is a transverse electromagnetic wave whose field oscillates perpendicular to the propagation direction. Light whose field vector E oscillates in only one direction is called linearly polarized light. The polarization direction is the direction in which the field vector E oscillates. The incident beam and reflected beam define the incident plane lying perpendicular to the interface of the two media. Light whose polarization plane lies perpendicular to the incident plane is called s-polarized light and light whose polarization plane lies parallel to the incident plane is called p-polarized light.
Tangential (s-)polarization is present when the light in the pupil of an optical system is linearly polarized and the polarization direction changes along the pupil so that the polarization direction is tangential to the optical axis (perpendicular to the radius vector) at every point of the pupil. In contrast, radial (p-)polarization is present when the polarization direction is radial to the optical axis (parallel to the radius vector) at every point in the pupil.
The current direction of the semiconductor industry favors the use of immersion systems for producing wafer structures smaller than 65 nm. Image-side numerical apertures of NA>1 are achieved by applying an immersion liquid to the wafer, so that smaller structures can be generated at the same wavelength. Accordingly, at a wavelength of λ=193 nm and when using water as immersion liquid, a maximum numerical aperture of 1.4 can be reached. Even greater numerical apertures can be achieved through the use of different immersion liquids. Therefore, with a reduction factor of 1:4, wafer structures of 65 m and 45 nm are required for mask structures of 260 nm and 180 nm, respectively. Since the mask structures are therefore in the range of the imaging wavelength (193 nm), the polarization effects due to the masks are also increasingly dominant.
When imaging a lithography mask through scanner systems, these p-polarized beam components are imaged differently than they are through an inspection microscope. Due to the magnified imaging of the lithography mask on a CCD matrix, the image-side numerical aperture is extremely small, so that vector effects do not occur. When a microscope is used to inspect lithography masks by emulating a scanner system, the occurring vector effects are absolutely necessary for examining the lithography mask because a realistic image of the scanner system cannot otherwise be emulated.
In the inspection microscopes known from the prior art, vector effects were not taken into consideration because the numerical apertures of the imaging systems that were used were less than 1 and vector effects therefore led to a minor error.
Therefore, the analysis of defects in the mask production process is increasingly important with smaller structures. For the last ten years, AIMS™ (Aerial Imaging Measurement System) by Carl Zeiss SMS GmbH has been commercially available for the analysis of mask defects with respect to printability. For this purpose, a small area of the mask (defect location) is illuminated and imaged under the same conditions of illumination and imaging (wavelength, NA, sigma, polarization) as with the lithographic scanner. In contrast to the scanner, however, the aerial image generated by the mask is magnified on a CCD camera. The camera sees the same image as the resist on the wafer. Therefore, the aerial image can be analyzed without wasteful test prints. Additional information for the analysis of the lithographic process window is obtained by taking a focus series.
While the lithography scanner images the mask structure in a reduced manner on the medium to be exposed, the structure is imaged in a magnified manner on a detector in mask inspections. The mask-side numerical aperture is the same in both systems.
According to the prior art, the image-side differences between the scanner and the emulation microscope are minor. As the image-side numerical aperture of the scanner increases, this difference increases and can no longer be ignored. This effect occurring in scanner imaging is called the vector effect.